The latest round of cumulative updates shipped to Windows 10 April 2018 Update PCs in October corrected a slew of bugs, but they also caused a curious one of their own. Microsoft has updated its release notes for the last two cumulative updates to roll out to the April update, noting a known issue that can prevent "some users" from setting Win32 program defaults for certain file types (via Windows Latest).

From the release notes:

After installing this update, some users cannot set Win32 program defaults for certain app and file type combinations using the Open with… command or Settings > Apps > Default apps.

In some cases, Microsoft Notepad or other Win32 programs cannot be set as the default.

Microsoft says that it is working on a fix that it expects to ship later in November. In the meantime, the only workaround is to keep trying to set app defaults. Apparently, sheer persistence occasionally will allow setting a default to succeed.